Characterization of host tolerance to Striga hermonthica

Summary One of the most promising control options against the parasitic weed Striga hermonthica is the use of crop varieties that combine resistance with high levels of tolerance. The aim of this study was to clarify the relation between Striga infestation level, Striga infection level and relative yield loss of sorghum and to use this insight for exploring the options for a proper screening procedure for tolerance. In three pot experiments, conducted in Mali (2003) and The Netherlands (2003, 2004), four sorghum genotypes were exposed to a range of Striga infestation levels, ranging from 0.0625 to 16 seeds cm⁻³. Observations included regular Striga emergence counts and sorghum grain yield at maturity. There were significant genotype, infestation and genotype × infestation effects on sorghum yield. The relation between infestation level and infection level was density dependent. Furthermore, the relation between Striga infection level and relative yield loss was non-linear, though for the most resistant genotype Framida only the linear part of the relation was obtained, as even at high infestation levels only moderate infection levels were achieved. The results suggest that for resistant genotypes, tolerance can best be quantified as a reduced relative yield loss per aboveground Striga plant, whereas for less resistant genotypes the maximum relative yield loss can best be used. Whether both expressions of tolerance are interrelated could not be resolved. Complications of screening for tolerance under field conditions are discussed.     

Partial resistance to neck blast influenced by stage of panicle development and rice genotype

Summary Neck nodes of eight rice genotypes were inoculated with a virulent isolate of the blast pathogen at four slightly different, increasingly older stages of panicle development shortly after flowering. Resistance to infection as well as resistance to growth of the pathogen after infection was assessed. Significant differences between genotypes were found for both components of resistance. Resistance to growth of the pathogen sharply increased with aging of the neck node as was indicated by a large reduction of the length of the lesions on the culm, but resistance to infection did not change much. The lesion length was closely related to yield loss. A small delay in time of infection can lead to a large decrease in yield reduction. Meaningful comparison of partial resistance to neck blast between genotypes therefore requires infection at an identical stage of panicle development. In the field, where panicles are in different stages of development, selection of genotypes that do show the symptoms of neck blast together with relatively little yield loss in infected panicles is recommended.     

Nuclear genetic control of variation in simazine tolerance in oilseed brassicas

Summary Substantial variation in tolerance to the triazine herbicide simazine was observed among a wide range of Brassica napus cultivars grown in a hydroponic screening system. None of these cultivars was as tolerant of are simazine treatment as the Canadian cultivar Triton which carries cytoplasmic gene(s) which confer a high level of resistance through their modification of chloroplast membranes to which triazine herbicides normally bind. A field study showed that the most tolerant cultivar Haya had a significantly higher yield than Triton in both handweeded plots and plots treated with 1000 ml/ha simazine. This finding supported other observations that the yield potential of cultivars such as Triton is substantially reduced because of the impairment of photosynthesis by cytoplasmic gene(s) for triazine resistance. A slight reduction in the yield of Haya with an increase in simazine concentration from 500 to 1000 ml/ha suggested that further increases in simazine concentration required for optimal weed control are likely to have a detrimental effect on the yield of Haya and other tolerant cultivars. Genetic analyses of variation in simazine tolerance in populations derived from crosses among cultivars representative of the range in tolerance indicated that the narrow-sense heritability of tolerance was of sufficient magnitude to allow for isolation of genotypes tolerant of higher simazine concentrations. These could be utilized in developing high yielding cultivars in areas of southern Australia where simazine treatment is necessary for effective weed control in Canola crops.     

Simple Sequence Repeat Markers for Detecting Sources of Tolerance to PTSL Syndrome in Prunus Persica Rootstocks

Summary Peach tree short life (PTSL) is a severe and complex disease syndrome in the Southeast of the United States that causes significant damage to peach orchards and affects more than 70% of the peach acreage in the region. The ring nematode is known to be one of the major factors strongly associated with the PTSL syndrome. A peach rootstock (Guardian^? Brand BY520-9, also referred to as Guardian^? or BY520-9), which is tolerant to ring nematode and PTSL, was released in 1994. To investigate the genetic relatedness of PTSL tolerant genotypes and identify possible parents of Guardian^? selections as sources of the PTSL tolerance trait, 98 genotypes derived from the BY520-9 pedigree, and others showing different degrees of field resistance or susceptibility to PTSL, were selected for simple sequence repeat analysis. Nineteen of 32 microsatellite loci were polymorphic. Allele number averaged 2.7 per locus with heterozygosity levels ranging from 0.07 to 0.60. Cluster analysis of these genotypes produced a dendrogram in agreement with their putative pedigrees. Thirty-eight F₄ genotypes from Guardian pedigree were used for the parental identification analyses. Of those, nine most likely parental genotypes were identified for 10 field tested F₅ genotypes produced from open-pollinated selections of BY520-9.     

Alternative methods of interpreting measurements of resistance to the leafhopper Empoasca kraemeri Ross and Moore in the common bean,Phaseolus vulgaris L.

Summary Appropriate definitions of, and methods of measuring, resistance are sought using Phaseolus vulgaris, a major source of protein in Latin America, and its most important pest, Empoasca kraemeri. The concepts of general vigour and narrow-sense resistance are defined: these two components make up broad-sense resistance. The validity of these definitions, which depend on the inclusion of tolerance as a resistance mechanism, is discussed. In order to distinguish the components, 38 genotypes of P. vulgaris were grown with and without insecticidal proctection, in three replications in a split-plot design, with three planting dates. Seed yield per plant, and a visual score of damage symptoms or vigour, were measured. Three methods of estimating narrow sense resistance are considered: 1) the genotype × insecticide interaction effect in a split-plots analysis, 2) the yield of each genotype unprotected (Y_u) divided by its yield protected (Y_p). and 3) the deviation of each genotype from a regression of Y_u on Y_p. The third method overcomes some objections to the first two, and can be applied to damage and vigour scores as well as to yields. The 38 genotypes differed in narrow-sense resistance according to all three methods, and it appears that in practice Methods 2 and 3 were both valid. They also differed in general vigour. Breeders working with many pests and crops have found that low damage and high yield in the presence of the pest are effective selection criteria. The work reported here supports this view.     

Resistance to Helminthosporium leaf blight and agronomic performance of spring wheat genotypes of diverse origins

Helminthosporium leaf blight (HLB), caused by a complex of Cochliobolus sativus (Ito & Kurib.) Drechsler ex Dastur and Pyrenophora tritici-repentis Died, is a serious disease of wheat (Triticum aestivum L.) in the warm lowlands of South Asia. Wheat cultivars grown in the area are either susceptible to HLB or possess low levels of resistance to it. A replicated field study was conducted in 1999 and 2000 at two sites in Nepal to determine the level of HLB resistance and other desirable traits in 60 wheat genotypes of diverse origin. The test genotypes were planted in main strips divided into two strips one of which was sprayed four times with Tilt (a.i. propiconazole) @ 125 g of a.i. ha^–1. Four readings of HLB were recorded to calculate the area under the disease progress curve (AUDPC). Other traits under investigation included biomass yield (BY), grain yield (GY), 1000-kernel weight (TKW), harvest index (HI), days to heading (DH) and maturity (DM), plant height (PHT), and effective tiller number (ETN). Wheat genotypes differed significantly for all traits. Mean AUDPC values ranged from 45 to 1268. A few exotic genotypes were highly resistant to HLB. Losses in GY due to HLB ranged from 2 to 26%, and TKW was reduced by up to 33%. A few genotypes showed HLB tolerance, i.e., relatively smaller GY and TKW reductions despite high levels of HLB. In general, medium to late maturity and higher levels of HLB resistance and low to high GY and TKW characterized genotypes exotic to South Asia. Biplot analysis identified several genotypes that were HLB-resistant and agronomically superior. Results suggest it is possible to improve HLB resistance of local wheat cultivars based on selective breeding using this pool of germplasm.     

Components of adult plant resistance to leaf rust in wheat

Summary Winter wheat genotypes were tested for resistance in the field by assessing the percentage sporulating leaf area after infection with wheat leaf rust. The disease level in the first field trial was too low for selection. In the second field trial a low sporulating leaf area was found on several genotypes showing a susceptible infection type. These genotypes possibly have partial resistance. Six genotypes possibly possess adult plant resistance, as they showed a resistant infection type and a low sporulating leaf area.The latency period, infection frequency and uredosorus size of sixteen genotypes were determined in the greenhouse after infection with two races of leaf rust at two temperature regimes. The temperature × genotype interaction, found for latency period and infection frequency, was mostly influenced by the cultivars Cerco, Tundra and Miller. Adult plant resistance was postulated for four genotypes whereas another four appeared to have partial resistance.Only one of the sixteen genotypes (Apexal) possessed adult plant resistance and two genotypes (Arminda and Cappelle Desprez) showed partial resistance in the field as well as in the greenhouse.     

Variation in resistance to different winter stress factors within a full-sib family of perennial ryegrass

Wintering ability in the field and resistance to different winter-stress factors under controlled environmental conditions were studied in a full-sib family of perennial ryegrass (Lolium perenne L.). Significant variation in tolerance to freezing and ice encasement, resistance to pink snow mould (Microdochium nivale) and also in winter survival and spring growth were found between the different genotypes. No strong correlations were found between the resistances to the different stress factors. These results indicate that resistance to different winter-stress factors is controlled by separate genes in perennial ryegrass. A low but significant positive correlation was found between spring growth of plants in the field after the first winter and both freezing tolerance and M. nivale resistance measured in controlled environments. Cold hardening seemed to influence freezing tolerance and M. nivale resistance differently in the different genotypes, since no distinct correlation in tolerance to freezing or resistance to M. nivale was found between unhardened and hardened plants. Tolerance or resistance to most of the winter stress factors measured was positively correlated with plant size. This revised version was published online in July 2006 with corrections to the Cover Date.     

Variation within and among Ladino white clover ecotypes for agronomic traits

Summary One set of 285 genotypes from three landraces and one natural population and a second set of 165 genotypes from two landraces and one natural population were evaluated in separate trials. Variation among populations was mostly accounted for by the difference between landrace and natural population genotype groups, the former showing features related to higher seed yield and short-term aerial dry matter (DM) yield and to lower persistence, cold tolerance and protein content. Compared to variance among ecotypes, that within ecotypes was never lower and it was at least four-fold higher for important traits such as aerial and total (aerial plus stolon) DM yield, stolon density, production of seed heads and cold tolerance. Landraces and natural populations showed similar levels of intra-population variation for all traits except cold tolerance, for which landraces tended to somewhat greater variation. Trait associations were less definite at the genotype level than what previously found in terms of mean values of the ecotypes. This finding and the relatively high broad sense heritability which was observed for all components of DM and seed yield can support the breeding activity on this clover. The level of variation and of heritability found for protein content do not suggest the routine evaluation of this trait in breeding programmes.     

Evaluation of proline analysis and chlorophyll fluorescence quenching measurements as drought tolerance indicators in durum wheat (Triticum turgidum L. var. durum)

Summary The possibility of using proline accumulation and fluorescence inhibition as predictive tests for drought tolerance in durum wheat has been investigated. The drought susceptibility of 25 genotypes was evaluated by comparing yields and yield components in irrigated and non irrigated conditions in the field. A drought susceptibility index (DSI) was calculated based on yields from irrigated and dry treatments and compared with the results obtained using the two physiological criteria. Proline accumulation and chlorophyll fluorescence inhibition were found to be significantly and negatively correlated with DSI of grain yield, biological yield, and thousand kernel weight, and tiller index. The use of both criteria for breeding durum wheat in Mediterranean dryland is discussed.     

Selection for improved adaptation of white clover to low phosphorus and acid soils

Summary A total of 490 white clover genotypes were grown in competition withAgrostis tenuis and repeatedly defoliated, in a glasshouse trial. The percentage change in shoot yield from the first to the last harvest (harvest 6) was negatively correlated with the soil-P level from which ecotypes were collected. High- and low-yielding genotypes were selected from this study and compared in a field trial on a low-P soil (12 mg Olsen P/kg soil), in a grazed mixed species sward. Over the first year there was no significant difference in spread into the sward, and over three years no significant difference in persistence. Selection in glasshouses for yield is unlikely to improve edaphic stress tolerance in the field.In a field study in New Zealand, comparison of six ecotype populations showed that after one year the highest-spreading population had been collected from a low-P soil while the lowest spreading population came from a high-P soil, although the correlation between spread and soi-P from which ecotypes were collected was not significant (r=–0.67 p>0.05). However, in another field trial in England it was shown that populations collected from low-P soils not only outyielded those, collected from high-P soils but that they also responded less to added P.It was concluded that populations collected from low-P soils will be a useful source of germplasm to identify genotypes adapted to low-P soils. Successful identification of such genotypes is most likely to be made in the field rather than in controlled more artificial environments.     

Characterization of resistance of cassava genotypes to bacterial blight by evaluation of leaf and systemic symptoms in relation to yield in different ecozones

Twenty-two improved and local cassava genotypes were evaluated for their bacterial blight symptom types in reaction to infection by Xanthomonas axonopodis pv. manihotis under field conditions in the forest, forest savanna transition and wet savanna zones of Togo. High genotype × environment interactions in development of each symptom type were observed. Combining data on environments and genotypes, spot, blight and wilt symptoms were positively correlated. Analysing genotype reactions across environments, indications for independent mechanisms of resistance on leaf and stem level, varying by genotype, were found. Genotypes Main27 with resistance to spot and blight symptoms and TMS4(2)1425 with resistance to wilt symptoms are recommended to breeders to introgress their resistance characteristics. Significant negative correlations were generally observed between blight and wilt symptom development and root yield across ecozones, with blight being more important under lower, and wilt under higher inoculum pressure. Genotypes TMS30572, CVTM4, TMS92/0429 and TMS91/02316 showed low spot, blight and wilt symptoms combined with high root yield across ecozones.     

Studies for assessing the influence of hardening on cold tolerance of barley genotypes

Summary Frost tolerance of 30 barley (Hordeum vulgare L.) cultivars have been field evaluated in North Italy during the 1990/1991 winter season that was characterized by exceptionally low temperatures without snow cover. The results showed a significant correlation between cold injury and grain yield loss (r=0.61^**). Five cultivars chosen for their varying degree of frost tolerance were further evaluated using laboratory tests. Measurements of survival rate and membrane damage were used to assess the influence of hardening on frost resistance. The reliability of the tests is shown by the high correlation to the field data. For both the laboratory temperature regimes and field conditions, the tested cultivars showed the same order of classification. The effect of a rise in temperature at the end of the hardening treatment on frost tolerance is also reported. The laboratory tests here proposed can be integrated in a breeding programme for improving frost tolerance in barley.     

Genetic variation for resistance to Fusarium head blight in bread wheat

Summary During a four year period, a total of 258 winter and spring wheat genotypes were evaluated for resistance to head blight after inoculation with Fusarium culmorum strain IPO 39-01. It was concluded that genetic variation for resistance is very large. Spring wheat genotypes which had been reported to be resistant to head blight caused by Fusarium graminearum were also resistant to F. culmorum. The resistant germplasm was divided into three gene pools: winter wheats from Eastern Europe, spring wheats from China/Japan and spring wheats from Brazil. In 32 winter wheat genotypes in 1987, and 54 winter wheat genotypes in 1989, the percentage yield reduction depended on the square root of percentage head blight with an average regression coefficient of 6.6. Heritability estimates indicated that for selection for Fusarium head blight resistance, visually assessed head blight was a better selection criterion than yield reduction.     

Partial resistance in lettuce to downy mildew (Bremia lactucae). 1. Search for partially resistant genotypes and the influence of certain plant characters and environments on the resistance level

Summary In three bicyclic inoculation experiments in the field 756 genotypes from the IVT Lactuca gene bank were screened for differences in partial resistance against Bremia lactucae. Significant differences between genotypes were found. No linkage was found between the presence or absence of R-genes and partial resistance nor was there a clear relationship between plant characters such as size, erectness and lettuce type and resistance level. Comparison of test results for genotypes which were tested twice and in different years and/or environments revealed a rather good relationship. So no clear genotype × environment interactions for resistance level were found. Results imply that the bicyclic inoculation experiments are useful to determine levels of partial resistance.     

Stability analysis over seasons and locations of multilines of wheat (Triticum aestivum L.)

Summary Three multilines each of Kalyansona and PV 18 varieties of wheat were compared for their stability of yield and agronomic characters and disease resistance against the respective recurrent parents. The experiments were conducted for four years at nine locations.The multilines had more tillers and bolder seeds than the recurrent parents. There were, however, no differences for plant height, ear length and number of spikelets per spike.The genotypes x years x locations interaction was much more important than genotypes x years or genotypes x locations interaction as well as the main effects for genotypes. It is suggested that the number of test locations should be increased while decreasing the number of years.The stability parameters indicated that the multilines had a higher mean yield in the case of Kalyansona multilines whereas the yield of multilines of PV 18 was not significantly inferior to that of the recurrent parent. The regression coefficients were very close to unity except for KSML 3 (b=1.132). The deviations from regression were much larger for the multilines than for the recurrent parents.For rust resistance the multilines were superior over the varieties Kalyansona and PV 18.This is publication No. 9 of the series Studies on multilines in wheat (Triticum aestivum L.)     

Early determination of genotypes for apple scab resistance by forced flowering of test cross progenies

Summary Apple selections with different major genes for resistance to apple scab (Venturia inaequalis) derived from Malus floribunda and M. pumila were crossed with each other. The progenies were screened as young seedlings for their reaction to V. inaequalis race 1. A gene for resistance from M. pumila, causing stellate necrotic (SN) lesions, was epistatic to a second gene for resistance from M. floribunda, causing irregular chlorotic (Chl) lesions. Although in most cases SN, Chl and susceptible phenotypes were clearly distinct, occasionally reactions were difficult to characterize or varied from one inoculation to another. Selected seedlings showing resistant or susceptible reactions were forced to flower in 16–20 months in the greenhouse and test crossed with susceptible cultivars. Test cross seedlings were screened for scab reaction. The presence of both genes for resistance in a resistant plant was indicated by presence of both Chl and SN resistant phenotypes in the test cross progeny. Chi-square analysis of four large progenies produced a good fit to the expected ratio. The use of the forced flowering technique to determine scab resistance genotypes in 28 months demonstrated its value in breeding apples with multiple disease resistance.     

Diallel analysis of sweetpotatoes for resistance to sweetpotato virus disease

Sweetpotato virus disease (SPVD) is due to the dual infection and synergistic interaction of Sweetpotato feathery mottle potyvirus (SPFMV) and Sweetpotato chlorotic stunt crinivirus(SPCSV), and causes up to 98% yield loss in sweetpotato in East Africa. This study was conducted to determine the inheritance of resistance to SPVD in sweetpotato and to estimate the nature of genetic variance. Ten parental clones varying in reaction to SPVD were crossed in a half diallel mating design to generate 45 full-sib families. The families were graft-inoculated with SPCSV and SPFMV to induce SPVD and evaluated for resistance in a randomized complete block design at two sites in Namulonge, Uganda during 1998–2000. In serological assays for SPFMV and SPCSV,resistance to symptom development and recovery from initial systemic SPVD symptoms, characterised resistant genotypes. Genetic component analysis showed significant effects for both general combining ability (GCA) and specific combining ability (SCA) for resistance to SPVD. GCA to SCA variance component ratios were large (0.51–0.87), hence GCA effects were more important than SCA effects. Resistant parents exhibited high GCA indicating that additive gene effects were predominant in the inheritance of resistance to SPVD and recovery. Narrow-sense heritability (31–41%) and broad-sense heritability (73–98%) were moderate to high, indicating that rapid genetic gains for SPVD resistance could be accomplished by mass selection breeding techniques. Two genotypes, New Kawogo and Sowola, had high negative GCA effects and had several families in specific crosses,which exhibited rapid recovery from SPVD,and are promising parents for enhancement of SPVD resistance and recovery. This revised version was published online in August 2006 with corrections to the Cover Date.     

Responses of wheat and barley genotypes to toxic concentrations of soil boron

Summary The growth and yield of seven wheat and two barley cultivars or lines, previously found to show different degrees of boron tolerance under field conditions, were compared in a pot experiment at a range of soil boron treatments. Soil treatments ranged up to 150 mg/kg applied B. Extractable B in soils ranged up to 103 mg/kg.At the highest B treatment seedling emergence was delayed, but the percentage emergence was not reduced. The degree of boron toxicity symptom expression varied between the wheat cultivars and lines, with the two most tolerant, Halberd and (Wq^*KP)^*WmH)/6/12, displaying the least symptoms.The concentration of boron applied to the soil which produced a significant depression of growth and yield varied between cultivars. For example, the yield of (Wq^*KP)^*WmH)/6/12 was not affected at the 100 mg/kg applied boron treatment, while the grain yield for (Wl^*MMC)/W1/10 was significantly reduced at the 25 mg/kg treatment.There was a linear increase in boron concentration in tillers at the boot-stage with increasing concentration of boron in the soil. The most boron tolerant genotypes had the lowest tissue boron concentrations in each of the treatments. Halberd and (Wq^*KP)^*WmH)/6/12 had approximately half the boron concentrations of the more sensitive genotypes at the 25 and 50 mg/kg treatments. Differential tolerance of boron within the tissue was also observed. Both Stirling and (Wl^*MMC)/W1/10 had significantly reduced total dry matter and grain yields at the 25 mg/kg treatment, while the concentrations of boron in boot stage tillers at this treatment were 118 and 100 mg/kg, respectively. On the other hand, Halberd and (Wq^*KP)^*WmH)/6/12 had tissue boron concentrations of 144 and 131 mg/kg, respectively, at the 50 mg/kg treatment but yield was unaffected.The relative responses in the pot experiment, for wheat, were in close agreement with field results. Halberd and (Wq^*KP)^*WmH)/6/12 had the highest grain yields, with the lowest concentrations of boron in the grain when grown under high boron conditions in the field. In pots these two genotypes proved to be the most tolerant of boron. For barley the advantage in grain yield in the field, expressed by WI-2584 compared with Stirling, was not repeated in pots. WI-2584 was, however, more tolerant than Stirling on the basis of total dry matter production.The results show that useful variation in boron tolerance exists among wheat, and that breeding should be able to provide cultivars tolerant to high levels of boron.     

High yielding spring bread wheat germplasm for global irrigated and rainfed production systems

Global wheat (Triticum aestivum L.) production must increase 2% annually until 2020 to meet future demands. Breeding wheat cultivars with increased grain yield potential, enhanced water-use efficiency, heat tolerance, end-use quality, and durable resistance to important diseases and pests can contribute to meet at least half of the desired production increases. The remaining half must come through better agronomic and soil management practices and incentive policies. Analyses of the recent International Yield Trials indicate that grain yields of the best new entries were usually 10% higher than the local checks globally, as well as within a country across sites. Variation in yield across sites within a country/region underline the role of genotype × environment (GE) interaction and provides opportunities to select for stable genotypes, which is not often done. The lack of proper analysis undermines proper utilization of germplasm with high yield potential and stability in the national wheat breeding programs. Some of the best performers in irrigated areas were amongst the best in semiarid environments, reinforcing the fact that high yield potential and drought tolerance can be improved simultaneously. The best performing lines often had genotypic base of widely adapted genotypes Kauz, Attila, Baviacora, and Pastor, with genetic contributions from other parents including synthetic wheat. We recommend within country multilocation analysis of trial performance for a crop season to identify lines suiting particular or different locations within a country. The immediate feedback on GE interaction will also help in breeding lines for countries having substantial variation across locations and years.